Offshore wind turbines are being developed that instead of resting on fixed-bottom support structures have a floating support structure.
Several configurations have been proposed for the floating or buoyancy structures: many of these employ floater elements in the form of hollow floater tanks that in use are arranged substantially below the mean sea level and provide a buoyancy force to support the wind turbine. Ballast and/or mooring lines anchored to the seabed are provided for achieving stability.
In some of these floating wind turbines, the buoyancy structure is designed to provide an excess buoyancy force and is maintained floating under the sea level by taut mooring lines tensioned by the excess buoyancy force.
For example, concepts have been developed such as the “Taught Leg Buoy” (TLB) floating wind turbine, with a slender cylindrical buoy and a plurality of tensioned mooring lines, inclined relative to the seabed and connected to gravity anchors and to the buoy; or such as the “Tension Leg Platform” (TLP) floating wind turbine, in which the tensioned mooring lines are substantially vertical and are connected between gravity anchors on the seabed and arms or braces extending radially outwardly with respect to the vertical axis of the wind turbine. The TLP arms may be part of the buoyancy structure, for example in the form of hollow spokes that extend radially outwardly from a hollow central hub, or may be arranged above the sea level, in which case the buoy may be a slender cylindrical tank like in the TLB concept.
The buoyancy structure of a floating offshore wind turbine is subjected to several loads, such as for example the weight of the wind turbine itself, impacts, forces exerted by waves, currents and tides, and, depending on the configuration of the wind turbine, also aerodynamic forces associated with the wind, rotor rotation, etc. In the presence of such loads floating wind turbines may have a tendency to destabilize.
In TLP configurations, the braces can suffer significant shear and bending forces due to the tension to which the mooring lines are subjected. These significant vertical forces require the braces to be very strong and thus heavy in order to suitably withstand them.
Moreover, these vertical forces may be subjected to some kind of oscillations due to other loads, such as those mentioned above (from waves, from wind, etc.). These oscillations may aggravate the effects of said vertical forces on the braces.
FIG. 1 schematically and partially shows a typical prior art TLP wind turbine comprising a floater tank 100, which provides an excess buoyancy force 110, and a tower 101 arranged on the floater tank 100. This wind turbine also comprises a plurality of arms 102 extending radially outwardly (with respect to a longitudinal axis 109 of the tower 101 and/or the tank 100) from a bottom region of the tank 100. Each arm 102 has an associated tensioned mooring line 105 having a first end 108 anchored to the seabed SB and a second end 107 attached to a distal region 103 (with respect to the axis 109) of the arm 102.
FIG. 1 shows how the tension to which a mooring line 105 is subjected causes a top-down force 106 which may be quite significant and may cause a significant bending stress on the corresponding arm 102. In particular, the proximal portion 104 of the arm 102 especially suffers this bending stress. This bending stress may require the arms 102 to have a very strong configuration which is normally based on large amounts of material. These strong configurations may thus be quite expensive.
There still exists a need for a new floating TLP wind turbine which at least partially reduces the abovementioned problems. It is an object of the present invention to fulfil such a need.